mysql_server component (sub-worklog for wl6667)

HLS:
===
What is the problem ?

MySQL has a platform independent character set implementation. To minimize
string conversions all strings in the server core have a character set attached
to them that's carried along as further as possible. This is done through a
special C++ class (String). Components may need to manipulate some of these
String instances. But due to the fact that C++ classes don't travel well across
heap boundaries (dll/exe etc) we need a C API for these class instances.

Why only this much ?

The full API for that would be very large. So this worklog implements only the
minimum subset that the password validation component (and plugin) needs.

How ?

We take the existing composite plugin service (a wrapper around String methods)
and break it up into multiple logically separate component services. This is a
typical example for how re-engineering of existing plugin APIs into service APIs
should go.